The present invention relates generally to turbines and particularly to land-based gas turbines for power generation employing closed-circuit steam-cooling paths for cooling the hot gas components. The invention more particularly relates to a cooling system for cooling a bearing of a turbine rotor and thermally insulating the bearing by inhibiting heat transfer to the bearing from a fluid medium passing through a bore tube assembly within the bearing and used for cooling the hot gas components of the turbine.
Cooling of hot gas path components, for example, the buckets of a gas turbine, has been proposed in. the past and found particularly effective in land-based power-generating plants. While gas turbines are typically air-cooled,. for example, jet engines employ compressor discharge air for cooling hot gas components, steam-cooling of land-based turbines is more efficient because the losses associated with the use of steam as a coolant are not as great as the losses realized by extracting compressor bleed air for cooling purposes. Also, in combined cycle operations, steam-cooling is particularly advantageous because the heat energy imparted to the steam as it cools the gas turbine components is recovered as useful work in driving the steam turbine in the combined-cycle operation.
In U.S. Pat. No. 5,593,274, of common assignee herewith, there is disclosed a gas turbine having coaxial steam passages for supplying cooling steam to hot gas components of the rotor, for example, the buckets, and returning spent cooling steam to a return. From that patent, it will be seen that the cooling steam path is coaxial within the aft shaft of the rotor. In a recent gas turbine design of the assignee of this application, the aft shaft extends rearwardly from and is integral with an aft shaft wheel coupled to the turbine wheel mounting the buckets of the final stage of the turbine. The aft shaft surrounds a bore tube assembly having, elongated outer and inner tubes spaced from one another and concentric about the rotor axis defining an annular passage for conveying a cooling medium, i.e., steam, in one axial direction to the buckets of the turbine wheels and conveying spent cooling medium in an opposite axial direction within the inner tube to a return. Further, the aft shaft wheel has a plurality of radially extending conduits for conveying the cooling steam of the supply passage radially outwardly to the rim of the rotor for conveyance to the buckets. Another set of conduits in the aft shaft wheel conveys spent cooling steam from the buckets and outer rim of the rotor radially inwardly to the inner passage to a return. The axial passages and radial conduits have significant thermal impact on the aft bearing because they naturally transfer heat via conduction, radiation, forced and natural convection to the bearing. However, there is a requirement that the bearing not exceed a predetermined maximum temperature and that the aft shaft cannot exceed a maximum temperature under the forward air/oil seal. Consequently, the present invention addresses the requirement for cooling the bearing and thermally insulating the bearing by inhibiting heat transfer from the cooling medium passing through the aft shaft and bearing in a manner such that these temperature limitations are not exceeded and ample margins are provided.
The present invention provides a cooling and. insulating system for inhibiting heat transfer from a cooling medium, e.g., steam, flowing through a bore tube assembly within the aft bearing of a gas turbine, to the aft shaft and aft bearing sufficiently that the operating temperature limitations of the aft bearing and the portions of the aft shaft under the air/oil seal have ample margin. The bore tube assembly comprises a pair of concentric tubes defining an annular coolant steam supply passage between the outer and inner concentric tubes and a spent cooling steam return passage within the inner tube. The bore tubes, as well as an aft shaft surrounding the bore tubes, form part of the rotating structure of the rotor. The forward end of the aft shaft comprises a radially projecting aft shaft wheel which is secured on the aft side of the wheel of the final-stage of the gas turbine. Conduits, in the nature of tubes, are disposed radially in the aft shaft wheel in communication with respective supply and return passages of the bore tube assembly. Consequently, the annular cooling steam supply passage supplies cooling steam to a set of radial conduits which, in turn, supply cooling steam to the rim of the rotor for delivery to the steam-cooled buckets in selected stages of the turbine. The other set of radial conduits in the aft shaft wheel conveys the hotter spent cooling steam from the steam-cooled buckets to the inner tube for return.
In order to cool the bearing and insulate it from the heat of the steam supply and return passages and conduits, a radiation shield is disposed about the outer tube and defines with the outer tube a dead air gap therebetween, thus insulating the aft shaft from direct conductive heat transfer from the outer tube to the shaft. In accordance with the present invention, there is provided a cooling fluid passageway between the heat shield and the inner surface of the aft shaft. The cooling fluid is preferably air, although other types of cooling fluids may be used, supplied from the cooling air in the aft bearing cavity or from a suitable source. The air is supplied through an in let aft of the aft bearing for flow forwardly through the passageway, thus providing an insulating and cooling flow of air, inhibiting heat transfer from the cooling steam to the aft sat
The cooling fluid passageway terminates axially forwardly of the aft bearing in a plurality Of radially projecting cooling pathways which empty the cooling fluid into a forward close-out cavity between the aft shaft wheel and the forward close-out plate. That close-out cavity lies in communication with the hot gas path at a location aft of the final bucket stage and radially outwardly of the inlet to the air fluid passageway.
A significant advantage of the present system resides in the use of existing turbomachinery to obtain a natural flow of cooling air through the passage and radially outwardly into the hot gas path. For example, the bearing cavity is typically cooled by cooling air supplied at a pressure above ambient pressure and, consequently, the cooling air.:in the bearing cavity flows naturally into the air fluid passageway. Additionally, the air inlet to the cooling fluid passageway lies on a radius inwardly of the air outlet. Accordingly, a pumping action occurs to pump the cooling fluid from the bearing cavity and along the passageway into the conduits for egress into the hot gas stream. Still further, the outlet for the air fluid passageway exits into the hot gas stream at a lower pressure than the pressure of the bearing cavity. Consequently, additional pumps or the like for flowing the cooling fluid are not necessary.
In a preferred embodiment according to the present invention, there is provided in a gas turbine having a rotor rotatable about an axis including a plurality of turbine wheels mounting turbine buckets, a rotatable bore tube assembly having elongated outer and inner tubes spaced from one another and concentric about the axis defining first and second passages for respectively conveying a cooling medium in one axial direction to the buckets of at least one of the turbine wheels and conveying spent cooling medium in an axial direction opposite one direction to a return, and an aft shaft about the bore tube assembly and rotatable therewith, and a bearing about the aft shaft, a cooling system for cooling the bearing, comprising an axially extending cooling passageway concentrically about the bore tube assembly between. the outer tube and the aft shaft and radially inwardly of the bearing, the system including an inlet for admitting a cooling fluid into the passageway for flow past the bearing and an outlet for receiving the cooling fluid whereby the cooling fluid cools the bearing.
In a further preferred embodiment according to the present invention, there is provided in a gas turbine having a rotor rotatable about an axis including a plurality of turbine wheels mounting turbine buckets, a rotatable bore tube assembly having elongated outer and inner tubes spaced from one another and concentric about the axis defining respective, first and second passages for respectively conveying a cooling medium in one axial direction to the buckets of at least one of the turbine wheels and conveying spent cooling medium in an axial direction opposite one direction to a return, and an aft shaft about the bore tube assembly and rotatable therewith, and a bearing about the aft shaft, a method of minimizing heat transfer from the cooling medium to the bearing, comprising the steps of providing an axially extending cooling passageway concentrically about the bore tube. assembly between the outer tube and the aft shaft and radially inwardly of the bearing and flowing a cooling fluid in the passageway past the bearing for minimizing transfer of heat from the cooling medium in the bore. tube assembly to the bearing.
Accordingly, it is a primary object of the present invention to provide a cooling and thermal insulating system and methods of operating the system for cooling the bearing of a gas turbine rotor by inhibiting heat transfer to the bearing from a cooling medium passing through the bearing.